The present invention relates to the making of seamless tubes and pipes with a diameter in excess of 200 mm by means of piercing solid rounds or round billets with a d=0.8 of the diameter of the tube or pipe to be made and also including sizing and smoothing of the pierced billet. In addition the invention relates to structure for making such tubes using two roll oblique rolling mills with stationary guides provided in between twin conical rolls and cooperating with an axially movable and adjustable conical mandrel, the mandrel having several sections that differ in diameter and conicity.
Methods generally and equipment of the kind to which the invention pertains have been known for some time. Since the invention of oblique rolling for piercing of solid rounds one has tried to manufacture directly sufficiently thin wall smooth tube by means of such mills. Unfortunately the practice has not yet been able to verify proposals along that line. Tn more than 100 years of development the manufacture of seamless tubing has always included a plurality of steps using stretching and the like or one provided for widening and expansion of the tube diameter for which particular devices and equipment was needed. Among that equipment for providing these tasks are, e.g. so-called pilgrim step rolls, but also plug mills, continuous longitudinal rolling machines etc.
Aside from the foregoing certain proposals have been made to improve the quality of the tubing to be made under utilization of oblique rolling. German Pat. No. 886,437 suggests that a so called reducing roll portion be interposed between the piercing and transverse rolling part within the oblique rolling mill, and the smoothing and the sizing portion of the mill on the other hand.
It is from a point of view of forming technology rather undesired to reduce the wall thickness in reducing mills while maintaining the diameter at the same value, and to provide in a subsequent smoothing part further rolling under utilization of a cylindrical mandrel, that acts against the kinematic conditions in that area. Also, it is believed that the manufacture of seamless tubes and pipes has not really been successfully practiced with any kind of oblique rolling mill, nor is it believed that more than two rolls can be used. This often remained an unconsidered fact so that the subsequent rolling procedure was necessary.